This invention relates to a magneto-optical recording medium for recording information by varying the state of magnetization of an information storage layer by raising its temperature by irradiating light to the recording medium including the information storage layer, and reproducing the information that is recorded by irradiating light to the above-mentioned recording medium by application of the magneto-optical effect, and also relates to a magneto-optical recording device using the above-mentioned recording medium.
Furthermore, this invention relates to a magneto-optical recording medium capable of overwriting the previously stored information with a single light beam, and also relates to a magneto-optical recording and reproducing device using the magneto-optical recording medium.
The ordinary magneto-optical recording medium of the prior art has a cross sectional structure shown in FIG. 13, for example. This magneto-optical recording medium is formed by successively laminating a dielectric layer 52 (about 90 nm thick) of a silicon nitride (SiN.sub.x) or the like, an information storage layer 53 (about 100 nm thick) of TbFeCo or the like and a protective layer 54 (about 200 nm thick) of silicon nitride or the like onto a transparent substrate 51 of glass or the like having formed therein grooves for tracking. The dielectric layer 52 causes the laser beam entering from the side of the substrate 51 to undergo multiple reflections inside the dielectric layer 52 and thereby increase the rotation angle (Kerr rotation angle) of the polarization plane of the reflected light produced at the information storage layer 53. The protective layer 54 protects the information storage layer 53 from corrosion, such as oxidation.
The coercive force of the information storage layer 53 has a temperature characteristic shown in FIG. 14. To be more specific, the coercive force is high at room temperature T.sub.R, and becomes infinity at compensation temperature T.sub.comp, and drops near the Curie temperature T.sub.c.
The principle of recording and reproducing information on this magneto-optical recording medium is described as follows.
An external magnetic field is first applied to this recording medium to set the magnetization of the information storage layer 53 uniformly in one direction. When recording information, while being applied to a recording magnetic field Hrec (in a direction opposite to the external magnetic field mentioned above) with an intensity shown in FIG. 14, the recording medium is irradiated with the laser beam in pulses. By this laser light irradiation, the temperature of the thus irradiated spots of the recording medium rises, and when the temperature exceeds a specified writing temperature Tw shown in FIG. 14, the coercive force of the information storage layer 53 becomes smaller than the recording magnetic field Hrec. Therefore, the magnetization of the information storage layer 53 mentioned above is reversed to the direction of the writing magnetic field Hrec, so that the recorded domains are formed.
To erase the information recorded as described, the direction of the above-mentioned external magnetic field is reversed and the laser beam is continuously irradiated to the recording medium.
When reproducing information, the laser beam of a lower power than in recording and erasing is irradiated to the recorded domains formed as described, and the Kerr rotation angle of the polarization plane of the reflected light is detected. Since the Kerr rotation angle varies with the presence or absence and the shape and the size of the recorded domains, the information is reproduced according to the Kerr rotation angles detected.
The above-mentioned conventional magneto-optical recording medium is incapable of what is called overwriting. For this reason, there has been proposed an overwritable magneto-optical recording medium which has a cross sectional structure shown in FIG. 15, for example. In this magneto-optical recording medium, a dielectric layer 52 (about 90 nm thick) of silicon nitride or the like is formed on a transparent substrate 51 of glass or the like having grooves for tracking, and onto this dielectric layer 52, a memory layer 53a (about 40 nm thick) of TbFeCo and a writing layer 53b (about 100 nm thick) of TbDyFeCo are laminated. The memory layer 53a and the writing layer 53b constitute an information storage layer 53. A protective layer 54 (about 200 nm thick) of silicon nitride or the like is formed on the writing layer 53b.
Since the thickness of the memory layer 53a is about 40 nm, only a small amount of the readout laser beam irradiated through the substrate 51 can reach the writing layer 53a. For this reason, the polarization plane of the reflected light rotates chiefly reflecting the state of magnetization of the memory layer 53a. The writing layer 53b, which is exchange-coupled magnetically to the memory layer 53a, is used for overwriting with a single beam.
The principle of overwriting on this magneto-optical recording medium will next be described with reference to FIGS. 16 and 17A to 17G. FIG. 16 is a graph showing temperature characteristics of the coercive forces of the memory layer 53a and the writing layer 53b, while FIGS. 17A-17G are diagrams for explaining changes in magnetization of the memory layer 53a and the writing layer 53b when overwriting is done.
As shown in FIG. 16, the temperature characteristics of the coercive forces were arranged such that the Curie temperature Tcm of the memory layer 53a is lower than the Curie temperature Tcr of the writing layer 53b, so that at room temperature T.sub.R, the coercive force of the writing layer 53b is smaller than the coercive force of the memory layer 53a. Hence, when an initializing field Hini with an intensity shown in FIG. 16 is applied by a permanent magnet or the like to the information storage layer 53 at room temperature T.sub.R, as is apparent from FIGS. 17A and 17B, the magnetization of the writing layer 53b can be directed uniformly in one direction without changing the direction (information) of the magnetization of the memory layer 53a.
When the laser beam of a relatively small intensity (low level) is irradiated to the recording medium having had the magnetization of the writing layer 53b directed uniformly in one direction and the recording medium is raised to a temperature close to the Curie temperature Tcm of the memory layer 53a, the coercive force of the memory layer 53a completely or substantially disappears. At this time, since the Curie temperature Tcr of the writing layer 53b is higher than the Curie temperature Tcm of the memory layer 53a, the direction of the magnetization of the writing layer 53b remains unchanged (FIG. 17C).
As the laser beam irradiation is finished and the temperature of the recording medium drops, the magnetization of the memory layer 53a appears, but the magnetization this time is directed by the exchange field to the same direction as the magnetization of the writing layer 53b (FIG. 17E). This state does not change even when the temperature falls further to room temperature T.sub.R. This process is what is called erasing of data.
On the other hand, if, while the recording field Hrec of an intensity shown in FIG. 16 is applied to the recording medium, the laser beam of a relatively high intensity (high level) is irradiated to the recording medium and the recording medium is raised to a temperature close to the Curie temperature Tcr of the writing layer 53b, then the coercive force of the memory layer 53a completely disappears and also the coercive force of the writing layer 53b completely or substantially completely disappears (FIG. D).
Under this condition, when the laser beam irradiation is finished and the temperature of recording medium falls a little, the magnetization of the writing layer 53b first appears. At this time, the direction of the magnetization of the writing layer 53b is reversed to the same direction as the direction of the recording field Hrec (FIG. 17F). As the temperature further falls and becomes lower than the Curie temperature Tcm of the memory layer 53a, the direction of the magnetization at this time becomes the same as the direction of the magnetization of the writing layer 32 (the same direction of the recording field Hrec (FIG. 17G). This state does not change even when room temperature T.sub.R is reached. This process is what is called recording of data.
As has been described, in the magneto-optical recording medium shown in FIG. 15, by modulating the laser beam intensity between high and low levels, the magnetized direction (information) of the memory layer 53a can be changed arbitrarily. In other words, overwriting can be performed with a single beam. This method is disclosed in JP-A-62-175948.
In the prior-art overwritable magneto-optical recording medium shown in FIG. 15, there is a problem that since the memory layer 53a has a low Curie temperature Tcm or has a thin thickness, the Kerr rotation angle of the reflected light is small and therefore, a satisfactory Signal to Noise ratio (SNR) cannot be obtained when reproducing information.
To increase the SNR in reproduction, it has been proposed to add a readout layer (copy layer) which provides the conventional magneto-optical recording medium with a large Kerr rotation angle (Refer to JP-A-63-64651, for example). In this case, however, a problem arises that if the readout layer is increased in thickness, owing to the effects of the magnetization of the readout layer, overwriting cannot be performed in an appropriate manner.
Moreover, in the conventional magneto-optical recording and reproducing method, there is a problem that if information is recorded or reproduced with high density (about 1.5 Gb/in.sup.2, for example), a sufficient SNR cannot be obtained.